The quantum well infrared photodetector (QWIP) has emerged as a mainstream technology for long wavelength infrared detection. Quantum Well Infrared Photodetectors (QWIPs) are infrared detectors that are made of layers of quantum well (QW) materials. These QW materials have a unique property that they are sensitive to light only when the light is propagating parallel to these layers, with which one of the electric polarizations EZ is pointing perpendicular to the layers. In other words, the QWIP material absorbs light only when the optical electric field is vertical to the material layers. The detector structure may be arranged in the form of large focal plane arrays (FPAs) of pixels.
Utilizing gallium arsenide material technology, QWIP focal plane array cameras are amenable for low cost and high volume production. QWIP cameras with resolution as high as 640×512 pixels are available in the commercial market. InGaAs/AlGaAs materials have proved to be useful in Quantum Well Infrared Photodetectors (QWIPs). InGaAs/AlGaAs materials have material properties that are highly reproducible and predictable by fundamental physical laws. For example, the observed material's absorption coefficient α for parallel propagating light is found to be in precise agreement with that calculated from quantum mechanics. Free from extrinsic factors such as material imperfection and surface leakage, the observed dark current also obeys the well-known thermionic emission model. With this simple and well-behaved material system, QWIP technology could have been developed and applied in a rapid pace. Unfortunately, the unique requirement for vertically polarized light has greatly impeded its development. Being sensitive only to the vertical polarization, Ez, the quantum efficiency (QE) of the detector is dependent on the pixel geometry, apart from its material properties.
When the QWIP is detecting light from a target area, light from the target area enters into the detector pixels normally (i.e., at a 90° angle). Because the QWIPs are sensitive to light only when the light is propagating parallel to the layers of the QWIPs, a reflective grating is conventionally placed on top of the pixels to scatter light, as shown in FIG. 1. In the presence of a grating with a particular period, the light with certain wavelengths will diffract at a large angle. Travelling at an oblique angle, the light can be partially absorbed by the material which generates photocurrent inside the pixel. The design of an optimal grating was described by J. Y. Anderson and L. Lundqvist in the article entitled “Grating-coupled quantum-well infrared detectors: Theory and performance”, J. Appl. Phys. vol. 71, pp. 3600-3610, 1992. Nevertheless, the general approach of using reflective gratings to couple light has achieved only very limited success so far, which precludes its application in demanding situations. To broaden its utility, a more efficient light coupling scheme is needed.
As reported in A. De Rossi et al., “Effects of finite pixel size on optical coupling in QWIPs,” Inf. Phys. and Tech., vol. 44, pp. 325-330, 2003, showed that when the pixel size is very small, diffraction from edges, rather than from the grating, dominates the optical coupling. The spectral response appears noticeably different from the ideal infinite grating, indicating a Fabry-Perot resonance between the pixel walls and between the grating and the air-substrate interface. Although large QE using the grating approach has been reported in J. Y. Andersson and L. Lundqvist, “Near-unity quantum efficiency of AlGaAs/GaAs quantum well infrared detectors using a waveguide with a doubly periodic grating coupler,” Appl. Phys. Lett., vol. 59, pp. 857-859, (1991), there is little evidence that high performance can be achieved in the high density, small pixel FPAs.
To improve the optical coupling in QWIPs, corrugated-quantum well infrared photodetector (C-QWIP) have been developed, as described in C. J. Chen et al., “Corrugated quantum well infrared photodetectors for normal incident light coupling,” Appl. Phys. Lett., vol. 69, pp. 1446-1448, 1996; K. K. Choi et al., “Corrugated quantum well infrared photodetectors for material characterization.” J. Appl. Phys., vol. 88, pp. 1612-1623, 2000. The “corrugated” terminology relates to using a number of V-grooves etched into the layered material to create angled mesa sidewalls for light coupling. Corrugated-QWIP utilizes optical reflections to change the direction of light inside the pixel. A C-QWIP pixel structure is shown in FIGS. 2A and 2B. In the C-QWIP structure shown in FIG. 2B, the inclined sidewalls reflect normal incident light into large angle propagation. The inclined sidewalls reflect light into parallel propagation and create the vertical field. The detector has a constant coupling efficiency when the detector substrate is thick. See, for example L. Yan et al., “Electromagnetic modeling of quantum-well photodetectors containing diffractive elements,” IEEE J. Quantum Electron., vol. 35, pp. 1870.1877, 1999; K. K. Choi et al., “Light coupling characteristics of corrugated quantum well infrared photodetectors,” IEEE J. of Quan. Electr., vol. 40, pp. 130-142, 2004. Corrugated Quantum Well Infrared Photodetectors (C-QWIP) cameras, for example, can be made in higher resolution, in larger production volume, at a lower cost, in higher sensitivity, in broadband and multi-color detection. The structure was patented by Choi in U.S. Pat. No. 5,485,015, hereby incorporated by reference, entitled “Quantum Grid Infrared Photodetector,” which discloses a quantum grid infrared photodetector (QGIP) that includes a semiconductor substrate with a quantum well infrared photodetector (QWIP) mounted thereon. Moreover, U.S. Pat. No. 7,217,926, issued May 15, 2007, hereby incorporated by reference, discloses “Systems involving voltage-tunable quantum-well infrared photodetectors.” For example, when using GaAs, having a refractive index is 3.34, the critical angle will be 17.4° when the GaAs material is in contact with air or vacuum. Since the sidewall angle is 50°, the angle of incidence for normal incident light will also be 50°, making it larger than the critical angle. The light will thus be totally internal reflected and be absorbed by the GaAs detector material.
By using C-QWIPs, because reflection is more effective in redirecting the light, the quantum efficiency (QE) is improved. Reflection is also independent of wavelength. This means that the detector will preserve the natural absorption spectrum of the material, which often can be much wider than the grating coupling bandwidth. Without the need for matching the material wavelength to the grating cavity modes in the detector, the same pixel geometry and production process are applicable to all QWIP material designs. This allows the simultaneous production of FPAs having a wide range of wavelength bands without jeopardizing QE. In the absence of the fine grating features, it also allows the use of standard photolithographic techniques for faster, less expensive and very large format production. However, there is a need for further improving QWIP technology in high-sensitivity and high-resolution imaging.